1. Field of the Invention
This invention relates to acoustic monitoring methods and systems in laser-induced optical breakdown (LIOB).
2. Background Art
Ultrafast lasers allow light to interact with materials in a femtosecond period, with peak powers many orders of magnitude higher than that of continuous wave light but with low average powers. Interestingly, an optically transparent material that has no linear absorption of incident laser light may have strong non-linear absorption under high intensity irradiation of a femtosecond pulsed laser. Non-linear absorption can lead to photodisruption of the material by generating a fast, expanding high-temperature plasma. Measurable secondary effects of the plasma include shock wave emission, temperature increases, and cavitation bubble generation. Many applications of ultrafast laser-induced optical breakdown (LIOB) have been developed recently, such as: micromachining of solid materials, microsurgery of tissues, and high-density optical data storage.
A number of methods have been developed to characterize LIOB. Stuart et al. determined LIOB via visual acquisition with Nomarski microscopy, which was simple but not well defined. Another approach for estimating breakdown threshold was to measure ablation depth using scanning electron microscopy (SEM). Furthermore, a combination of different microscopy techniques including optical microscopy, atomic force microscopy and SEM has been employed for accurate characterization of LIOB. However, none of these methods are real-time or applicable to liquid or liquid-like samples.
The dominant breakdown attributes studied in liquids are shock-wave emission and cavitation bubble generation. As a shock wave propagates spherically outward from the laser's focus, it dissipates energy and can be considered a broadband pressure wave after propagating only a few wavelengths from the source. Hence, pressure sensors can be strategically positioned within the liquid to record acoustic events associated with each optical breakdown. To observe cavitation bubble formation and subsequent behavior, laser-flash photography, optical limiting, and third-harmonic generation (THG) techniques are often employed. Each looks at a specific and limited facet of an optical breakdown.
U.S. Pat. Nos. 5,615,675 and 5,732,046 disclose opto-acoustic transducers for internally examining objects.
Laser-induced optical breakdown (LIOB) with femtosecond pulsed lasers is utilized in diverse applications, including biomedical systems, material characterization, and data storage. LIOB parameters and behavior have been investigated extensively. It occurs when sufficiently high threshold fluence is attained at the laser focus, inducing plasma formation. Plasma formation leads to non-linear energy absorption and measurable secondary effects that include shock-wave emission, heat transfer, and cavitation bubbles (i.e., photodisruption). The presence and magnitude of these breakdown attributes are used to determine a material's LIOB threshold.
U.S. Pat. No. 6,471,968 discloses a multi-functional nano device platform in the form of a dendrimer complex. Dendrimers are highly branched spherical macromolecules that provide templates for guest molecules to form dendrimer nanocomposite (DNC) particles. Optical limiting and third-harmonic generation techniques can explore some non-linear optical properties of these particles and their aggregates; yet they provide only limited information about the photodisruption.